1) For Template-Based and existing VM-based VGP deployments, we cannot currently modify the Current Disk Size but we can add new disks to the VM which will be created in addition to the Disk from the Template/VM.
2) For "VMware Virtual Machine" (virtual bare metal) deploys, we can specify number and sizes of disks.
Another option might be to use a BLPackage deploy job to resize the existing disk right after the VGP deploy job completes. At a high level, the steps there would be:
a) Create a BLPackage from the Hard Disk custom object. This can be any disk which is currently seen from the Live Browse.
b) Once the BLpackage is created we will have to change the Name, Disk File, Disk Size, Thin Provisioned attributes in the BLPackage. This operation is to be done for every disk we add to or modify on a VM. The Disk File will be the key thing here to specify under which VM this VM will be added.
c) Once this is done we will have to deploy this package against the vc server.
Whether the BLPackage adds a new disk or resizes an existing one, is determined by the Action value (Add or Modify) In the Modify case you will have to give the Disk Path and Disk name properly pointing to the current files of Disk. Using this technique, we can only increase the Disk Size, we cannot decrease it.
There is an RFE logged for this functionality. QM001779494. I am not sure if the required VMWare APIS exist to allow for this right now - the RFE is currently under consideration.